Ignition system



Jan. 20, 1970 J. B. FARR 3,490,426

IGNITION SYSTEM 7 Filed July 20, 1967 2 Sheets-Sheet l INVENTOR JAMES B, FARR ATTORNEYS Jan. 20, 1970 FARR 3,490,426

IGNITION SYSTEM Filed July 20, 1967 I 2 Sheets-Sheet 2 I I I O O VI IP b o w I L! \L \I; I 92 92' /I C V: x

I 98 VGT I i UL AHGULAR Posmoug TIME. {3 ma ANGULAR POSITION I-ITIME FIG. 6 FIG. 5 INVENTOR JAMES B. FARR ATTORNEYS United States Patent 3,490,426 IGNITION SYSTEM James B. Farr, Washtenaw County, Ann Arbor, Mich., as-

signor to Tecumseh Products Company, Tecumseh, Mich., a corporation of Michigan Filed July 20, 1967, Ser. No. 654,860 Int. Cl. F02b 75/04, 75/36; Hb 37/02 US. Cl. 123-149 9 Claims ABSTRACT OF THE DISCLOSURE A single-cylinder internal combustion engine is provided with a breakerless magneto ignition system. The magneto includes a permanent magnet mounted on the engine flywheel and a main coil and a triggering coil mounted in spaced relation on the stator assembly. During each revolution of the flywheel, a capacitor in the ignition circuit is charged from the main coil through a rectifier and the primary winding of an ignition transformer. After the magnet has passed the main coil, capacitor discharge is initiated by a triggering signal generated in the triggering coil by the magnet. The capacitor discharge paths are arranged to obtain a damped oscillatory discharge over multiple cycles. For one charge polarity, the capacitor is discharged and recharged to the opposite polarity through a silicon controlled rectifier. For the opposite charge polarity, the capacitor is discharged and recharged to the initial polarity through a Zener diode in its forward direction. The Zener diode is connected in the circuit so that the reverse breakdown of the diode limits the maximum voltage applied to the rectifier, the capacitor and the silicon controlled rectifier. Additionally, the Zener diode suppresses negative voltages generated in the main coil to protect the rectifier, the silicon controlled rectifier, the capacitor and the main coil of the stator against excessive voltage in the reverse direction.

A magneto ignition system employing breaker points is conventional on single cylinder engines. Breaker point ignition systems leave much to be desired and have many disadvantages including poor engine performance with fouled spark plugs, nonuniform voltage output and thus nonuniform sparking and engine performance with speed variations; breaker point deterioration due to arcing and repeated opening and closing; and adjustment and replacement of the breaker points.

It is highly desirable to improve the reliability, operation and performance of ignition systems used with single-cylinder engines and overcome the aforementioned disadvantages. Solid state ignition systems, including capacitor discharge ignition systems, have been the subject of much development in connection with automotive applications and the many advantages of this type of ignition system are well known. However, prior art capacitor discharge systems for automobile ignitions require modification for best results with single cylinder engines and for compatibility with a magneto charging source. Moreover, such prior automotive systems would be too expensive in relation to the relatively low cost of a single cylinder engine. It is essential that the cost of an ignition system for a single cylinder engine be held to a minimum due to vigorous competition in the small engine field.

The objects of the present invention include providing an ignition system that is particularly suited for singlecylinder engines having a magneto; that is breakerless and thus eliminates the disadvantages of breaker points in prior ignition systems; that is simple in construction, relatively inexpensive and reliable; and that provides better performance with fouled spark plugs and uniform sparking over a wide range of engine speeds.

Other objects, features and advantages of the present invention will be apparent in connection with the following description, the appended claim and the accompanying drawings in which:

FIG. 1 is a fragmentary view, partly broken away and in section, of a single cylinder engine having a flywheel magneto and a capacitor discharge ignition system of the present invention;

FIG. 2 is a view diagrammatically illustrating the rotor and stator of the flywheel magneto of FIG. 1;

FIG. 3 is a circuit diagram of the ignition system;

FIG. 3a illustrates a voltage regulator circuit that can be susbtituted for a Zener diode in the circuit illustrated in FIG. 3;

FIGS. 4a-d illustrate various wave forms associated with the circuit of FIG. 3 during the charging of the capacitor in the ignition system;

FIGS. Sa-e illustrate various wave forms that occur in the circuit of FIG. 3 during discharge of the capacitor; and

FIG. 6 illustrates a modification of the circuit in FIG. 3 to provide temperature stabilization.

Referring more particularly to the drawings, the breakerless magneto ignition of the present invention is applied to a single cylinder engine 10 partly shown in FIG. 1. Engine 10 includes the usual cylinder 12 and crankcase 14 having a crankshaft v16 journalled at one end in a bearing plate 18 which closes an end Wall on the crankcase 14. A magneto 20 generally comprises a finned flywheel 22 and a stator plate 24. Flywheel 22 is generally in the shape of an inverted cup and is keyed on an extension 26 of the crankshaft 16 outwardly of crankcase 14. Embedded in the side wall of flywheel 22 is a permanent magnet 28 having a north pole 30 and a south pole 32 (FIG. 2) disposed at the inner periphery of the flywheel. The flywheel 22 and magnet 28 serve as the rotor for the magneto 20.

The stator plate 24 is rigidly fastened to the bearing plate 18 and is provided with a central aperture 34 through which the crankshaft extension 26 projects. Mounted on one side of the stator plate 24 is a field coil assembly 36 which includes a coil 38 wound on the cen ter leg 40 of an E-shaped core 42. Coil 38 serves as the source of charging current for the capacitor in the ignition system of the present invention. The construction and orientation of the E-shaped core 42 and the permanent magnet 28 are substantially similar to that of corresponding elements in certain conventional breaker point ignition systems. This arrangement provides a rapid flux reversal through the center leg 40 and thus a relatively high voltage across output terminals 44, 46 of the coil 38. A potted circuit assembly 50 is also mounted on plate 24 spaced from the field coil assembly 36 peripherally of flywheel 22. The potted assembly 50 generally includes the circuit components enclosed within dotted lines in FIG. 3. Within the potted assembly 50 is a trigger coil 52 which is wound on a core 54 adjustably fastened on the stator plate 24. Coil 52 serves to initiate the spark during each revolution of the flywheel 22 as will be explained in greater detail hereinafter. The coil 52 and core 54 are disposed adjacent the inner periphery of flywheel 22 so that flux from the magnet 28 will link the coil 52 as the magnet passes the coil. Engine timing is controlled by the circumferential position of coil 52 according to crankshaft position during the compression stroke of the engine. In the preferred embodiment, the stator plate 24 is constructed to be interchangeable with the corresponding stator plate in a breaker point system without modification of the flywheel. Flywheel 22 rotates in a clockwise direction relative to stator plate 24 as designated by the arrow in FIG. 2.

Referring to the circuit of FIG. 3, a Zener diode 58 is connected directly across terminals 44, 46. Diode 58 is poled in its forward direction to short coil 38 when terminal 44 is negative relative to terminal 46. For positive voltage at 44 relative to terminal 46, the reverse breakdown threshold of diode 58 will clamp the maximum positive voltage for the ignition circuit. Also connected across terminals 44, 46 is a first series circuit comprising a silicon diode 60, a capacitor 62 and the primary winding 64 of a step-up transformer 66. The secondary winding 68 of transformer 66 is connected directly through a high tension lead 70 to a spark plug 72 for cylinder 12. Connected directly across the series connected capacitor 62 and winding 64 is a silicon controlled rectifier 74 having an anode 76, a cathode 78 and a gate 80. The trigger coil 52 is connected to gate 80 and cathode 78 to initiate conduction of rectifier 74 in response to magnet 28.

The function and operation of the breakerless magneto ignition system described hereinabove can best be understood in connection with the wave forms illustrated in FIGS. 4 and wherein the angular positions of crankshaft 16 are plotted along the abscissa axes and the magnitudes of the various wave forms, including flux, voltage and current, are plotted along the ordinate axes as will be described. The abscissa axes can also be considered as representing time. It will be understood that the Wave forms are for purposes of illustration and explanation and they are not necessarily intended to be scale. In FIG. 5, the time or position axes are expanded considerably over the corresponding axes in FIG. 4 since the capacitor discharge (FIG. 5) occurs over a relatively short time by comparison to the duration of the charging voltage and the triggering voltage (FIG. 4). With the engine running, as magnet 28 sweeps past core 42, the flux through coil 38 will vary as shown by the wave form 90 (FIG. 4a) with the maximum positive rate of flux change occurring when the magnet is centered over the core in the position shown in FIG. 2. The voltage generated in coil 38 (open circuit) is illustrated by a voltage wave form 91 (FIG. 4b) and includes a small negative pulse 92 followed by a relatively large positive pulse 93 which is in turn followed by a second small negative pulse 92'. This alternating voltage 91 is clamped at a low value in the forward direction of the Zener diode 58 and regulated at the reverse breakdown voltage of diode 58 in the re verse direction. Thus, the negative voltage pulses 92, 92' are substantially suppressed and the maximum voltage for the positive pulse 92 is limited. At high speeds the negative pulses 92, 92' become quite large and without clamping by diode 58 expensive rectifiers (diode 60 and controlled rectifier 74) would be required to withstand a high reverse voltage. By limiting the maximum positive pulse 93, substantially uniform capacitor charging is obtained over a wide range of engine speeds. Relatively inexpensive components, including the diode 60, the controlled rectifier 74, the capacitor 62 and the transformer 66 can be used because diode 58 provides the combined clamping and limiting.

The positive voltage 93 across terminals 44, 46 is coupled through diode across the capacitor 62 and winding 64 to charge capacitor 62 as illustrated by the capacitor voltage Wave form 96 (FIG. 4c). Capacitor 62 charges rapidly because the inductance of winding 64 offers practically no resistance at the relatively low frequency of the charging voltage. Capacitor 62 cannot discharge back through coil 38 due to diode 60 and thus the charge is held at the polarity indicated in FIG. 3 as the flywheel 22 and magnet 28 continue to rotate. When magnet 28 approaches coil 52, a voltage V represented by the wave form 98 (FIG. 4e) is generated in the coil and applied to the gate 80 of rectifier 74. As this voltage V 98, swings positive, rectifier 74 is gated on at a time designated t in FIGS. 4 and 5. The time t is selected according to the engine cycle so that rectifier 74 is gated on when the piston approaches top center during the compression stroke. By way of example, on one 3.5 HP. engine t was set at 18 before top center at 1000 r.p.m. Gate remains forward biased for substantially the remainder of the positive half cycle of the gating voltage 98.

Capacitor 62 begins to discharge rapidly through rectifier 74 and the primary winding 64 as illustrated by the first half cycle 100 on the capacitor voltage discharge wave form 102 (FIG. 5a). Capacitor 62 is discharged from the polarity shown in FIG. 2 and recharged in the opposite direction through rectifier 74 due to the inductance of Winding 64. When capacitor 74 is completely charged to the opposite polarity and the current drops to zero, rectifier 74 is reverse biased at its anode and cathode and it stops conducting at a time t Capacitor 62 then discharges and recharges to its original polarity through the series path including winding 64, diode 58 and diode 60. FIG. 5b illustrates the voltage wave form 104 at the controlled rectifier 74 and the capacitor discharge current is illustrated by the wave form 108 (FIG. 50). In FIGS. 5a, 50, the capacitor voltage and current, respectively, are shown in full lines while rectifier 74 is conducting and in broken lines while diode 58 is conducting. Discharge of capacitor 62 continues alternately via rectifier 74 and diode 58 so that the capacitor fully discharges in the damped oscillatory manner illustrated in FIGS. 5a and Sc. The duration of the positive gating voltage 98 greatly exceeds the time required to fully discharge capacitor 62 to assure the full wave discharge over several complete cycles. By way of illustration, the positive gating voltage may have a duration on the order of 1000 ,usec. whereas the time required for capacitor 62 to discharge fully may be on the order of 50 1sec. The alternating discharge current through winding 64 produces a stepped up secondary voltage sufiicient to ionize the spark plug gap at plug 72. The spark plug current 112 and the spark plug voltage 114 are shown in FIGS. 4d and 4e, respectively. The duration of the spark is determined primarily by the damping resistance in the trans former windings 64, 68, the capacitor 62 and the spark gap. The spark is extinguished after several full cycles when the current in the secondary becomes insuificient to maintain the gap ionized. The point at which the spark ceases will be at a current reversal point as shown in FIG. 5d. In general, for a small-horsepower, single-cylinder engine, a spark duration of three cycles provides very satisfactory performance. The spark duration can be increased by increasing the primary leakage inductance of transformer 66, or by increasing the capacitance of capacitor 62 or by decreasing the damping resistances in the primary and secondary circuits.

To insure the desired full wave discharge, the displacement 0 (FIGS. 2- and 4) between coils 38, 52 is preferably selected so that capacitor 62 is fully charged and pulse 92 has been dissipated before capacitor discharge is initiated. Stated differently, the circumferential displacement 0 between coil 38 and coil 52 relative to the angular extent or circumferential length of magnet 28 and of core 42 is such that no appreciable flux links core 42 when magnet 28 reaches coil 52. This separation results when the circumferential spacing between core 42 and coil 52 is substantially greater than the circumferential distance between the outer ends of the poles 30, 32. By time separating the initial capacitor charging from the capacitor discharge, the required alternating conduction of rectifier 74 and diode 58 and thus several full cycles of capacitor discharge are obtained without any complication by voltages being generated in the magneto coil 38. The capacitor fully discharges long before the next charging voltage is generated in coil 38 during the next revolution of flywheel 22.

One of the more important features of the circuit is the provision of the Zener diode 58 to accomplish several important functions. To summarize, diode 58 allows capacitor 62 to discharge through the primary winding 64 over several full cycles and therefore provide multiple cycles of s ark plug current during each spark. Diode 58 suppresses negative voltages generated in coil 38 and thereby prevents excessive reverse voltages on diode 60. The reverse breakdown of diode 58 regulates the initial charge on capacitor 62 to provide uniform sparking over a wide range of engine speeds. With the positive voltage at terminals 44, 46 regulated to a reasonable value of say 150 volts, good protection is provided for diode 60 and the controlled rectifier 74. Rectifier 60 and controlled rectifier 74 together with capacitor 62 and transformer 66 can be relatively inexpensive items since they do not have to withstand high voltages that would otherwise be present at high engine speeds if diode 58 were not used.

FIG. 3a illustrates a simple modification for the circuit of FIG. 3 wherein the diode 58 (FIG. 3) is replaced by a diode 120 to provide one of the discharge paths for capacitor 62. The voltage regulation function of diode 58 (FIG. 3) is provided in the modification (FIG. 3a) by a series resistor 122 and a neon lamp 12 connected across diode 120.

FIG. 6 illustrates modifications in the circuit of FIG. 3 to improve stability particularly at high operating temperatures. A silicon diode 138 is connected in series between the cathode 78 of the controlled rectifier 74 and the output terminal 46. Connected across coil 52 is a voltage divider comprising a resistor 140 and a thermistor 142. Thermistor 142 has a negative temperature coefficient so that its resistance decreases with increasing temperature. The gate 80 is connected to the divider between resistor 140 and thermistor 142. Stray flux in the magneto tends to generate undesirable ripple voltages in the trigger coil 52 which would trigger rectifier 74 at an improper crankshaft angle. This undesirable ripple voltage is even more likely to cause spurious triggering with the engine operating at high temperatures. The silicon diode 138 connected in series with the cathode 78 greatly improve the stability at high temperatures. Anodecathode leakage current provides a small voltage drop across diode 138 so that the gate-cathode junction becomes reverse biased at higher temperatures, for example, temperatures above 110 C. The ripple voltage must overcome the reverse bias before the rectifier 74 triggers. It has been found that good circuit stability is extended up to 150 C. with diode 138. Additional stability is provided by thermistor 142. With increasing temperature, the gate voltage developed across the thermistor drops otf. Temperature compensation using either thermistor 142 or diode 138, alone, is also contemplated.

It will be understood that the breakerless magneto ignition system has been described hereinabove for purposes of illustration and is not intended to indicate limits of the present invention the scope of which is defined in the following claims.

I claim:

1. An ignition system of the capacitor discharge type for igniting a combustible charge in an internal combustion engine having at least one spark device therein, comprising a source of electrical energy, a capacitor, a first series circuit operatively coupled across said source for charging said capacitor and including an asymmetrical conducting device and a transformer primary winding, a transformer secondary winding operatively connected to said primary Winding and adapted for connection to said spark device, an electron control device having a main current conducting path and further having a control input for controlling conduction in said main path, trigger means operatively coupled to said control input to initiate conduction in said main path in accordance with predetermined timing of said engine, a second series circuit for discharging said capacitor in one direction through said primary winding when said capacitor is charged at one polarity and comprising said capacitor, said primary winding and said main current path of said electron control device, circuit means having first and second terminals and providing a first low impedance path between said terminals for current in a first forward direction therethrough and providing a high impedance path between said terminals in a reverse direction for applied reverse voltages less than a critical reverse, Zener-type breakdown voltage, said circuit means further providing a second low impedance path in said reverse direction between said terminals when applied reverse voltages are greater than said critical voltage, and wherein said ignition system further comprises a third seres circuit for discharging said capacitor in an opposite direction when said capacitor is charged in an opposite direction when said capacitor is charged at an opposite polarity and including said capacitor, said primary winding, said first low impedance path of said circuit means and said asymmetrical conducting device so that opposite polarity half cycle current flows in said primary winding during discharge of said capacitor via said second and said third series circuits and said second low impedance path provides protection against high reverse voltages in said first series circuit and regulates said one polarity charge on said capacitor.

2. The ignition system set forth in claim 1 wherein said source has a pair of output terminals, said circuit means is connected directly across said output terminals so as to present its high impedance path across said output terminals for one voltage polarity across said output terminals, and said first series circuit is connected directly across said output terminals to charge said capacitor through said asymmetrical conducting device for said one voltage polarity across said output terminals.

3. The ignition system set forth in claim 2 wherein said circuit means comprises a Zener diode.

4. The ignition system set forth in claim 1 wherein said electron control device comprises a controlled rectifier having an anode, a cathode and a gate electrode, said trigger means has a pair of output terminals one of which is connected to said gate electrode, and wherein a second asymmetrical conducting device is connected between said cathode and the other output terminal of said triggering means so that anode-cathode leakage current will pass through said second asymmetrical conducting device to thereby reverse bias said gate electrode in accordance with variations of said leakage current gaused by temperature variations at said controlled recti- 5. The ignition system set forth in claim 1 wherein said source comprises a magneto having a rotor member and a stator member, a permanent magnet carried on one of said members and first coil means carried on the other of said members to serve as said electrical energy source, said trigger means comprises second coil means carried on said other of said members circumferentially displaced from said first coil means, said first coil means has first and second output terminals, said first coil means and said magnet are arranged so that an alternating current is developed across said output terminals in response to relative rotation between said stator and said rotor, said circuit means comprises a Zener diode connected across said output terminals to regulate maximum voltage across said terminals at one voltage polarity and provide said first low impedance path across said terminals at the opposite voltage polarity to thereby provide a clamped pulsating unidirectional voltage across said output terminals, said asymmetrical conducting device comprises a diode rectifier having a first terminal connected to one of said output terminals, said capacitor and said primary winding being connected in series with each other between a second terminal of said diode rectifier and the other of said output terminals, and wherein said electron control device comprises a controlled rectifier having a pair of main electrodes and a control electrode, a first of said main electrodes being connected to said second terminal of said diode rectifier and a second of said main electrodes being connected to the other of said output terminals.

6. The ignition system set forth in claim 1 wherein said source has first and second output terminals providing an alternating current there-between, said circuit means comprises a Zener diode connected across said output terminals to regulate maximum voltage across said terminals at one voltage polarity and provide said first low impedance path across said terminals at the opposite voltage polarity to thereby provide a clamped pulsating unidirectional voltage across said output terminals, said asymmetrical conducting device comprises a diode rectifier having a first terminal connected to one of said output terminals, said capacitor and said primary winding being connected in series with each other between a second terminal of said diode rectifier and the other of said output terminals, and wherein said electron control device comprises a controlled rectifier having a pair of main electrodes and a control electrode, a first of said main electrodes being connected to said second terminal of said diode rectifier and a second of said main electrodes being connected to the other of said output terminals.

7. An ignition system of the capacitor discharge type for igniting a combustible charge in an internal combustion engine having at least one spark device therein, comprising a source of electrical energy, a capacitor, 2. first series circuit operatively coupled across said source for charging said capacitor and including an asymmetrical conducting device and a transformer primary winding, a transformer secondary winding operatively connected to said primary winding and adapted for connection to said spark device, an electron control device having a main current conducting path and further having a control input for controlling conduction in said main path, trigger means operatively coupled to said control input to initiate conduction in said main path in accordance with predetermined timing of said engine, a second series circuit for discharging said capacitor in one direction when said capacitor is charged at one polarity and comprising said capacitor, said primary winding and said main current path of said electron control device, circuit means having first and second terminals and providing a first low impedance path between said ter- .in an opposite direction when said capacitor is charged at an opposite polarity and including said capacitor, said primary winding and said first low impedance path of said circuit means whereby opposite polarity half cycle current flows in said primary winding during discharge of said capacitor, and wherein said electron control device comprises a controlled rectifier having an anode, a cathode and gate electrode, said trigger means has a pair of output terminals one of which is connected to said cathode, and a voltage divider connected across said terminals of said trigger means and comprising a first impedance means and a second impedance means, said first impedance means being variable in response to temperature variations and said gate electrode being connected to said divider at a point electrically between said first and said second impedance means.

8. An ignition system for igniting a combustible charge in. an internal combustion engine comprising a magneto having a rotor member and a stator member, said rotor member being adapted to be driven in synchronism with said engine relative to said stator member, a permanent magnet carried on one of said members so that its magnetic field encounters the other of said members during relative rotation thereof, first coil means carried on the other of said members, second coil means carried on said other member angularly spaced from said first coil means so that during each revolution of said rotor member said magnet sequentially generates a first electrical signal in said first coil means and a second electrical signal in said second coil means, said magnet and said coil means being arranged on said members so that said first electrical signal has a duration related to a predetermined portion of one revolution of said rotor, said magnet and said second coil means being arranged so that said second electrical signal has a duration related to a second portion of a rotor revolution which is time separated from said first portion of a rotor revolution, and wherein said ignition system further comprises a capacitor, first circuit means including a diode rectifier operatively coupled to said first co-il means and to said capacitor to charge said capacitor in response to said first signal, a transformer having a primary winding and a secondary winding, and a second circuit means operatively connected to said capacitor, said primary winding and said second coil means and responsive to said second signal to discharge said capacitor through said primary winding, said second circuit means having a first branch circuit including a controlled rectifier to discharge said capacitor at one polarity thereof and recharge said capacitor to an opposite polarity and a second branch circuit including said diode rectifier and a Zener diode to discharge said capacitor at said opposite polarity and recharge said capacitor to its original polarity, and said controlled rectifier being coupled to said second coil and responsive to said second signal to initiate discharge of said capacitor through said one branch circuit during said second portion of a rotor revolution.

9. The ignition system set forth in claim 8 wherein said controlled rectifier has a control input connected to said second coil and is responsive to a predetermined value of said second signal to initiate conduction through said main path, and said angular spacing between said first and said second coil means is selected so that during one revolution of said rotor said capacitor is fully charged via said first signal before said second signal reaches said predetermined value.

References Cited UNITED STATES PATENTS 3,037,148 5/1962 Gayler 123l49 3,152,281 10/1964 Robbins. 3,240,198 3/1966 London et al. 3,326,199 6/1967 McMillen. 3,327,165 6/ 1967 Hawthorne 315-223 3,358,665 12/1967 Carmichael et 3.1. 3,367,314 2/1968 Hirosawa et a1. 2,980,093 4/1961 Short. 3,249,808 5/1966 Short 3l5219 LAURENCE M. GOODRIDGE, Primary Examiner 

